A quadrupole mass analyzer includes a quadrupole unit 80, which is, as shown in FIG. 4, composed of four rod electrodes 81, 82, 83, 84 placed in parallel to and symmetrically around the z axis. A direct current (DC) voltage U and a high frequency (normally a radio frequency RF) alternate current (AC) voltage V.multidot.cos(.omega..multidot.t) are simultaneously applied between a pair of electrodes 81 and 83 placed along the x axis and the other pair of electrodes 82 and 84 placed along the y axis. When ions are introduced into the center of an end of the quadrupole unit 80 while the RF/DC voltage is applied, only ions 88 having a specific mass/electric charge ratio (m/z) according to the values of the voltage U and V can pass through the quadrupole unit 80 but other ions 87 disperse. Thus the quadrupole unit 80 is used as a mass filter by setting appropriate values of the voltage U and V, and the mass of the filtered ions can be scanned by changing the values of U and V.
As shown in FIG. 5, an MS/MS type mass analyzer includes three quadrupole units (Q.sub.1, Q.sub.2, Q.sub.3) placed in line between an ion source 11 and an ion detector 13. An object sample is ionized in the ion source 11 and the ions of various masses are introduced into the first quadrupole unit Q.sub.1. The first quadrupole unit Q.sub.1 allows ions of a preset mass M.sub.P to pass therethrough and to enter the second quadrupole unit Q.sub.2. The second quadrupole unit Q.sub.2 is accommodated in a case called collision chamber 12 in which collision gas such as Ar or N.sub.2 is contained. The ions 14 that have passed through the first quadrupole unit Q.sub.1 (which are then referred to as "parent ions") collide with the collision gas molecules and dissociate into partial ions (which are then referred to as "daughter ions"). The daughter ions 15 thus generated are conveyed by the electric field of the second quadrupole unit Q.sub.2 to the third quadrupole unit Q.sub.3. The third quadrupole unit Q.sub.3 functions similarly to the first quadrupole unit Q.sub.1 and allows daughter ions 15 of a preset mass M.sub.41 to pass therethrough and to enter the ion detector 13.
As described above, a direct current (DC) voltage U and a high frequency (or RF) alternate voltage V.multidot.cos(.omega..multidot.t) are simultaneously applied between two rod electrode pairs in each of the three quadrupole units Q.sub.1 -Q.sub.3. The DC voltage U and the RF voltage V.multidot.cos(.omega..multidot.t) are generated by a driver circuit 86 (FIG. 4). Aside from the driver circuit 86, a bias DC circuit 85 is provided to apply a bias DC voltage between the ion source 11 and the quadrupole unit 80. The bias DC voltage accelerates the ions generated by the ion source 11 to adequately pass the ions through the quadrupole unit and, for the second quadrupole unit Q.sub.2 of the MS/MS type mass analyzer, to give the ions enough collision energy to adequately dissociate. The bias DC voltage is applied to each of the three quadrupole units Q.sub.1, Q.sub.2 and Q.sub.3, and, as shown in FIG. 5, the values of the bias DC voltage V.sub.1, V.sub.2 or V.sub.3 depend on purposes of the respective quadrupole units Q.sub.1, Q.sub.2 and Q.sub.3.
A problem in the prior art MS/MS type mass analyzers is that when the frequency of the RF voltage applied to adjacent quadrupole units differs slightly or there is a subtle phase mismatch between them, a beat occurs between them which disturbs and disperses the ions passing through the adjacent quadrupole units. In this case, naturally, lighter ions are influenced more.